There has been conventionally known a mechanism in which a reinforcing lower frame disposed to extend between lower portions of back-part side frames of a seat back part is deformed rearward at the time of a rear-end collision to absorb an impact applied to the seat back part (refer to Patent Document 1). Further, as in Patent Document 2, there is known a structure in which coupling members couple reclining mechanism parts disposed on both left and right sides, thereby exhibiting predetermined strength against a side collision in addition to an impact absorbing function at the time of a rear-end collision.